1. Field of Invention
The invention relates to a backlight illumination system for illuminating a large area with light of a uniform color. In addition, the invention relates to a display device comprising such a backlight illumination system.
2. Description of Related Art
When light sources of different color are used for creating a uniform illumination system, a problem of achieving good color uniformity arises. Not only the number of light emitters of each individual color must be selected properly but the light emitters must, in addition, be placed in an optimal pattern in order to avoid disturbing local color variations. The backlight illuminating system may have a special color mixing arrangement, such as, e.g., in a so-called edge-lit backlight illumination system for a liquid crystal display (LCD) device, the backlight being provided with a color mixing light guide. In another situation, the backlight illuminating system may have a relatively poor color mixing arrangement, such as, for example, in a direct backlight illuminating system. In yet another situation, the backlight illuminating system may have virtually no color mixing arrangement, such as, for example, in an edge-lit backlight illumination system for a LCD, the backlight not being provided with means for additional color mixing. Both types of backlight illumination systems require dedicated design rules. The light emitters can be light sources of essentially different color, such as, for example red (R), green (G) or blue (B) light sources. In another situation, the light emitters may comprise light sources with a color which was meant to be the same but came out slightly different due to, for example, the fabrication process variations. Examples of such a situation can be phosphor-converted light-emitting diodes (LEDs) with varied thickness of phosphor layer or multi-dice RGB white packages with varied brightness of individual dices.
In patent application WO A 02 090 826 a backlight illumination system for illuminating a display device is described comprising a light-emitting panel with a front wall, a rear wall and edge areas. At least one of the edge areas is light transmitting and associated with a light source. The light comprises symmetric clusters of light-emitting diodes (LEDs) having three mutually different light-emission color points, for example symmetric clusters of blue, green and red LEDs.
A disadvantage of the known backlight illumination system is that the light and color distribution of the backlight illumination system is insufficiently uniform. As a result the illumination of a large area with light of a uniform color is insufficient.